The two main deviations of eye alignment are called tropia and phoria. Tropia (also called strabismus, squint, crossed eyes), is a deviation where the subject cannot point the fovea of both eyes at the same object simultaneously. An exotropia is when one eye constantly points outward, and an esotropia is when one eye constantly points inwards. Similarly, a hypertropia or hypotropia occur when one eye constantly points upwards or downwards. The second type of deviation, phoria, is a latent deviation that is only present when one eye is covered. When both eyes are open, the subject is able to point both eyes at the same object. If an exophoria is present, then the eye that is covered will turn outwards until the cover is removed. Then, it will take up fixation on the same object as the other eye. For an esophoria, the eye that is covered will turn inwards until the cover is removed. For hyperphorias or hypophorias, the eye that is under cover will point upwards or downwards relative to the eye that is not covered. Both tropias and phorias are a source of double vision, discomfort, and can cause difficulty with reading. A tropia can cause permanent vision loss called amblyopia or lazy eye in children. These conditions can be congenital and/or genetic, or acquired through traumatic brain injuries. Both tropia and phoria are established medical conditions that are readily treatable when they are identified. The earlier in life that a tropia is identified, the more likely it is that treatment will reverse/prevent permanent vision loss.
Clinical measurements of tropia and phoria are used across multiple healthcare fields to detect vision problems, either naturally-occurring or due to traumatic brain injury, that would lead to double vision. The predominant, current method for measuring eye alignment, called the cover test, is manual, technically-difficult, and tedious. Other widely-used clinical methods that are automated only determine whether tropia is present, but these methods do not detect the more common deviation in alignment, phoria.
All current methods of measuring eye alignment, either manual or automated, also lack the ability to detect whether or not the subject is accommodating, or focusing the eyes as if to look at an object closer than optical infinity. It is useful for the individual measuring eye alignment to know whether or not someone is accommodating because over- or under-accommodating during a tropia or phoria measurement affects the lateral position of the eye, i.e., how much the eyes are turned inwards or outwards.
Therefore, methods, apparatus and systems are desired that improve the detection and treatment of blinding and debilitating eye alignment disorders and that overcome challenges in the art, some of which are described above.